Components which operate under conditions of friction or abrasion are traditionally made of antifrictional alloys (cast iron, bronze). Alternatively, structural alloys, chrome- or nickel-base metallic or compound coatings are applied to the surfaces of the components. In the latter case, this has a hardening effect on the surface. However, as with the use of antifrictional alloys, the abrasion resistance parameters stay low because of the insufficient hardness of the friction surfaces. This leads to a quick abrasion of the expensive components and makes it necessary to periodically change them during their period of use.
Vansovskaya describes an electrochemical method to generate a hard and abrasion-resistant coating. Vansovskaya, G. A.: "Galvanitcheskie pokrytiya" (Galvanic coatings), Moskva, Mashinostroenie, 1984, p. 78. This method consists in applying a chrome layer of a certain thickness to the surface of a component which operates under conditions of abrasion. The method is characterized by the use of an aggressive and toxic electrolyte (chromic anhydride) and a high current density (up to 60 A/dm.sup.2). These are crucial for the conditions under which the technological process itself is being conducted as well as for the quality of the preliminary processing of the surface. The slightest deviations lead to a weak cohesion of the coating with the surface of the component to which the coating is applied and as a result of this, to the exfoliation during the period of use.
SU 1783004 describes a method for microplasma oxidation of valve metals and their alloys, mainly aluminum and titanium. Avtorskoe svidetelstvo SSSR 5 1783004, published in 1992. For this method an aqueous solution of electrolytes, containing phosphate, borate, and tungsten alkali metal is used. In the beginning of the processing of the surface, a voltage is applied (up to 360 V), during which a coating begins to form. During this process the current density is maintained constant (0.1 A/cm.sup.2). The given voltage and current parameters are maintained for a period of 1 to 3 minutes and the voltage is then decreased to zero over a 11/2 minute period.
The presented method is characterized by a series of restrictions in terms of the result that is achieved; these restrictions are the following:
it is practically impossible to generate thick and abrasion-resistant coatings; and PA1 there are considerable energy expenditures during the process of applying the coatings to the relatively large surfaces. The above-mentioned insufficiencies restrict a wider application of the technique.
The most similar method in terms of the underlying technology is an electrochemical microarc technique of applying silicate coatings to aluminum components. Patent of the Russian Federation 2065895, published in 1996. With this technique, the components, which are to be treated, are stepwise--in 4 to 7 cycles--immersed in an electrolytic bath with a sodium silicate, polyphosphate and arzamite-base electrolyte. Here, in the beginning of the process, when the components are being immersed in the electrolytic bath, an initial current density in the range of 5 to 25 A/dm.sup.2 is applied to only 5 to 10 % of their total surface area and maintained constant during the following stepwise immersion. The main insufficiencies of this method are the following:
1. The complexity of the process, as it is necessary to organize the stepwise immersing and the controlling of the surface area of the components which are immersed in the electrolyte, and also to control and regulate the required current density level;
2. The coatings which are generated have a relatively low abrasion resistance, due to the chemical nature of the used electrolyte as well as the technological operations being conducted; and
3. The method can only be used for the application of coatings to aluminum components. A change in the nature of the metal and of the chemical composition does not allow to generate high-quality coatings in terms of abrasion resistance and corrosion resistance parameters. These insufficiencies prevent a wider acceptance of the method.